swap_pager: small improvement to find_least

[dougm]

Summary

This change drops an unneeded test, a branch and avoids a needless computation. Altogether, it saves 10 bytes of instructions on amd64.

It will mean that, on the path to MPASS(0), the same lookup and iteration will happen twice.

Test Plan

The kernel still boots.

[kib]

sys/vm/swap_pager.c

2279

2312

Perhaps it is the time to change these two lines into __unreachable().